Patchcord length measurement for LAN testers

ABSTRACT

A LAN tester has display and remote units each having a connector jack attached to an adapter board for connection to the plug of a patch cord. Both the display and remote units have circuits which are capable of measuring the phase between a drive signal voltage and the corresponding coupled or reflected signal due to the drive signal. Scattering parameters for the mated connector pairs and the patch cord itself are measured during a field calibration. A computer in one or both of the tester units stores the measured scattering parameters and uses the scattering parameters to move the reference plane to any desired location along the patch cord. The length of the patch cord can be determined through the use of phase by measuring the frequency difference between adjacent maxima of a plot of input impedance versus frequency.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/432,858, filed Dec. 12, 2002 and U.S.application Ser. No. 10/317,555 filed Dec. 12, 2002.

BACKGROUND OF THE INVENTION

[0002] Local area network (LAN) cabling is used to connect equipmentsuch as personal computers, printers and fax machines that passinformation between them using high-speed digital signals. This type ofhigh performance cabling is sometimes referred to as telecommunicationscable. Since an office contains many computers, computer file servers,printers, and fax machines, the LAN cabling interconnects all of thisequipment into a communications network. LAN cabling has been designedto support telecommunication between all of the individual elements ofthe network.

[0003]FIG. 1 shows an example of LAN cabling, in a simplified drawing.FIG. 1 shows how the LAN cabling, most of which runs within the buildingwalls, is used to connect the personal computer 1 at someone's desk tothe file server 2 in the telecommunications room. The maximum length ofcable 3 inside the wall cannot exceed 90 meters. Wall jack connectors 4are used to connect the cords 5 from the computer and file server to theLAN cabling.

[0004] Cabling: Cabling is an important word in the term LAN cablingbecause cabling includes the connectors 4 placed on the LAN cable aswell as the cable 3 itself. Thus, the performance of the LAN cablingdepends upon the connectors as well as the cable.

[0005] Installation: Technicians install the LAN cabling as a part ofnew construction or as part of a LAN performance upgrade in existingstructures. In either case, the technicians cable 3 through the wallsand then place the connecting jacks 4 on the ends of the cable. Thejacks are then snapped into the wall jack mounting plate and theinstallation is complete.

[0006] However, the technician is then required to test each LAN cablingrun or link with calibrated test equipment. This testing certifies tothe general contractor that the cabling run has been correctly installedfrom the standpoint of signal integrity. Hand-held LAN testers are usedto perform these tests. The testers drive the cabling with a series ofdifferent signal types and from measurements of the received signals,determine if the cabling is capable of supporting the telecommunicationsignals at the prescribed data rate.

[0007] The LAN testers record the results of each test and, at a latertime, print out a test document indicating that the link passed orfailed. The technician gets paid for the links that pass. If there arelinks that fail, the technician must re-test, and often replaceconnectors that have been incorrectly or improperly installed. Thetechnicians keep testing and repairing the links until they all pass.

[0008] LAN Testers: LAN testers are fairly sophisticated hand-held testsystems, which can test LAN links with a series of tests covering afrequency range of 1 to 250 MHz, in the case of TIA Category 6 cabling.FIG. 2 shows a typical LAN tester 6, with a test adapter circuit board 7connected to the LAN tester. The test adapter circuit board includes atest jack connector 8. The purpose of this test adapter is to provide aconnection interface between the LAN tester and the LAN link to betested.

[0009] The test jack 8 allows the LAN tester 6 to connect to the LANlink with a patch cord 9, as shown in FIGS. 3 and 4. Typical lengths forpatch cords are two meters, or approximately six feet. This lengthallows the technician to conveniently connect the LAN tester to the walljacks 4 during test runs.

[0010] Standards: Technicians test their installed links with referenceto telecommunication industry standards. In the United States thestandard is specified by the TIA or Telecommunications IndustryAssociation. In Europe the standard comes from ISO, or InternationalStandards Organization. When testing a link, the technician selectswhich type of link is being tested and the corresponding sets ofmeasurement limits, whether from TIA or ISO.

[0011] The link is tested and the measured results are compared tolimits from the specified standard. If no limits are exceeded, the linkpasses. If not, then the link fails and the technician must work on thefailed link, as required, until it passes. Often this means reinstallingthe connectors on the ends of the cable.

[0012] Standard Link Definitions: FIG. 5 shows the standard permanentlink, in simplified form, with 90 meters of LAN cable, running within astructure's wall, or overhead in the ceiling. The wall jacks, attachedto the cabling ends, are used to connect the link with equipment in thetelecommunications room and to individual items such as computers orprinters within the office's local area network. The TIA and ISO specifythe length of 90 meters as the maximum length for the permanent link.

[0013] Link Testing: FIG. 6 illustrates how the LAN testers check theperformance of a link. When testing a link (a procedure known in theindustry as “shooting” a link), two LAN testers are required as shown.The technician connects a display end LAN tester 6A at one end of thelink, and the remote end LAN tester 6B at the other end of the link.Since the display end LAN tester has a display screen to show themeasurement test results, the technician shoots the link from thedisplay end, controlling the test from there, and viewing the testresults.

[0014] During the test, first one unit applies test signals to one endof the link while both units measure the results. Then the roles arereversed with the signal application and signal measurement taking placeat the opposite ends of the link. When the test is complete, the remoteunit sends its data measurement files to the display unit for finalprocessing and storage within the display unit. The limits for eachtest, specified by the selected standard, are applied to the measurementdata set to determine if the link passed or failed the certificationtest.

[0015] Standard Links: Both the TIA and ISO have defined two types ofLAN links, the channel link and the permanent link. Each link is shownand discussed below.

[0016] Channel Link: The channel link includes the LAN link and thepatch cords, as shown in FIG. 7, but does not include the connection tothe channel test adapter boards 7A. The channel link measurement pathincludes the link 3 inside the walls, the mated connector pairs at thewalls and the patch cords and is supposed to represent the performanceof the final, complete telecommunications link, which also uses patchcords to connect the personal computers and file servers to each other.Since there is a longer length of cabling in this path, the test limitsfor the channel link are not as stringent as those for the permanentlink.

[0017] Permanent Link: The permanent link includes the link 3, plus themated connector pairs at the wall jack, but it does not include thepatch cord, as shown in FIG. 8. Nor does it include the connection tothe permanent link test adapter board 7B. The permanent link testevaluates only the cable within the walls, the connector jacks at thewall, the plugs that are inserted into the jacks, and two centimeters ofcable that is attached to each of the plugs. The permanent link testessentially represents the performance of just the link cabling withinthe walls. Consequently, the permanent link test limits are the tightestmeasurement limits to pass.

[0018] As a result, technicians are often told that if their link failsthe permanent link test, to change over the LAN tester limits to channellink limits and re-test. If the channel test passes, the link may thenbe considered to pass under these conditions.

[0019] Consideration will now be given to the test issues faced by thetechnicians as they test their installed Local Area Network (LAN)cabling for compliance with the appropriate TIA or ISO measurement testlimits. The technician will certify the installed link to eitherpermanent or channel link measurement limits. It is assumed that thetechnician has performed steps necessary to calibrate the test equipmentin the field before the LAN certification test to assure maximum LANtester measurement accuracy.

[0020] Permanent Link Testing Issues

[0021] 1. Permanent Link Adapter Construction: Note the prior artpermanent link test adapters 7B shown in FIG. 8. Keep in mind thepermanent link comprises the cable in the wall plus the mated connectorpair at the wall jacks, but it does not include most of the patch cord.The permanent link adapters (PLA's) are typically fabricated by cuttinga patch cord in half, and then soldering each of the cut patch cord endsto a printed circuit board (PCB) within the permanent link test adapterhousing. These PCB's are designed to cause very little signal integrityproblems so that their effects are ignored.

[0022] 2. Permanent Link Testing Lifetime: Permanent link adapters havea limited test lifetime due to mechanical flexing of the patch cord asit enters the PLA housing. When the patch cord has been flexed beyondits maximum number of flexures, it will require replacement. When thishappens, the entire PLA has to be replaced. In addition, for maximumtest accuracy, both PLA's, the one at the display end and the one at theremote end should be replaced.

[0023] 3. Dedicated PLA: The LAN testers often use a dedicated PLA foreach permanent link tested. This is because the circuit and transmissionline properties of the patch cord can be an important part of theoverall PLA measurement result. The installation technician needs to beaware of what link he or she is testing, who made the cabling, and whatis the preferred type of PLA to use.

[0024] 4. Matched PLA Sets: Usually the technician will use a set ofPLA's matched to the cable type, by vendor, which is used in the link.If the link is made with cabling, (that is, cable plus connectors), fromVendor X, then a PLA made from Vendor X patch cords will be used for thecertification test.

[0025] 5. PLA Cost: The PLA's can be a costly item for the installers,often $400 or more for a set of two. If the LAN cabling installationtesting company has several installers, each requiring several differentsets of vendor specific PLA's, this overhead item can be rather costly.The cost comes from a dedicated printed circuit board, within a plastichousing, to form the structure of the PLA, which connects to the LANtester.

[0026] 6. PLA Cross-talk: In addition, as LAN certification moves tofrequencies above 250 MHz, the performance of the PLA's as a part of themeasurement system becomes more critical. The measured cross talk orlack of isolation between conductor pairs within the PLA connectioncircuit board becomes a serious issue as frequencies increase. When theisolation degrades beyond a certain level, the LAN tester cannot measurethe cabling pair-to-pair isolation because it cannot “see” past its ownPLA generated crosstalk.

[0027] The present invention provides the solution to this problem. Thesolution is to use a connector with proven isolation properties on thetest adapter board, and then to connect to that test adapter board witha patch cord having a connector which mates to the connector on theadapter board.

[0028] 7. PLA Reference Plane Calibration: The last issue with permanentlink adapters is that of the measurement reference plane location. Thepurpose of permanent link calibration is to refer all permanent linkmeasurements to a known point along the patch cord. In particular, thepermanent link measurement reference plane is calculated to set thispoint at the end of the patch cord, 2 centimeters from the wall jack.From this calibration, all effects from the patch cord are removed fromthe permanent link measurement. The calibration procedure used to defineand set this reference plane at this point can involve taking an initialset of permanent link calibration data and finally referring it to thisdesired reference plane.

[0029] Channel Link Testing Issues

[0030] 1. Channel Link Adapters: Note the channel link test adapters 7Ashown in FIG. 7. Keep in mind the channel link includes the link (i.e.,the cable in the wall plus the mated connector pairs at the wall jacks)and the patch cords but it does not include either the plugs or thejacks at the channel test adapter boards. The channel link adapters(CLA's) are fabricated by placing a right-angle connector withappropriate isolation on the printed circuit board mounted within theCLA housing. The right angle connector is selected to providesignificant pair-to-pair isolation when mated with the patch cord usedfor the channel link certification.

[0031] 2. CLA Testing Lifetime: Channel link adapters have a much longertest lifetime when compared to permanent link test adapters since theuse of low cost replaceable patch cords solves the patch cord mechanicalflexure problem. The connector mounted on the printed circuit boardinside the CLA eventually wears out as the cladding on the contactswears off. Nevertheless, the testing lifetime for the channel linkadapter is considerably longer than that for the permanent link adapter.

[0032] 3. Dedicated CLA: The LAN testers also use a dedicated CLA whentesting channel links since low cross talk, high isolation connectors 8are used on the channel link adapter printed circuit board.

[0033] 4. Matched CLA Sets: Matched CLA sets are used by definition byvirtue of the high isolation right angle printed circuit boardconnectors mounted on the PCB within the CLA housing. However, whencompared to the PLA, any type of patch cord can be used with the CLA, solong as the patch cord is compliant with the cabling category used forthe link under test.

[0034] 5. CLA Cost: The CLA's are less costly than PLA's, since they canuse any compliant patch cord to connect to and test the channel link.

[0035] 6. CLA Cross-talk: The channel link pair-to-pair isolation issuperior to that of the permanent link by virtue of the low crosstalkconnector used within the CLA module housing.

[0036] 7. CLA Reference Plane Calibration: The last issue with channellink adapters is also that of the measurement reference plane location.In particular, the channel link measurement reference plane is set atthe end of the patch cord connector right at the input end of the patchcord, as shown in FIG. 7. With this calibration, all effects from thepatch cord input connector (i.e., the plug at the tester end) areremoved from the channel link measurement.

[0037] LAN Link Measurement Issue Summary

[0038] From the preceding discussion, when compared to channel linkadapters, permanent link measurements require the use of a separate setof permanent link adapters, which add an undesirable set of costs interms of: 1) the permanent link adapters themselves; 2) the number ofdedicated PLA sets; and 3) limited PLA test lifetime due to patch cordflexure failure. Permanent link adapters also have more problems withminimizing pair-to-pair crosstalk when compared to channel linkadapters.

SUMMARY OF THE INVENTION

[0039] For these reasons, in the present invention acalibration/measurement method is proposed the objectives of which areto:

[0040] 1. eliminate completely the permanent link test adapter;

[0041] 2. reduce LAN measurement overhead support costs;

[0042] 3. improve signal integrity;

[0043] 4. increase LAN link measurement accuracy at frequencies above300 MHz;

[0044] 5. provide a means to measure permanent links using channeladapters and low cost patch cords; and

[0045] 6. provide a means to measure the physical length of a patchcord.

[0046] Phase

[0047] Preparatory to a description of the method of the presentinvention, a discussion of phase needs to be presented. The capabilityof phase measurement is a key attribute of the LAN tester of the presentinvention. That is, in addition to magnitude, the hand-held LAN testerof this invention can measure phase. This capability permits the testerto set a measurement reference plane at one specified point along theLAN link to be measured. The original calibration reference plane may beset at a point along the link at a point, which is easy to set, measure,define and implement.

[0048] Phase also allows the tester to easily move this originalcalibration reference plane and all of its associated LAN linkmeasurements to another, new, reference plane location at any timeduring the LAN link testing. Specifically, with phase information, adisplay end and remote end can each move the phase reference plane fromwithin the channel link adapter printed circuit board, through the matedpair of connectors at the CLA output and anywhere down the length of thepatch cord, and up to the mated pair of connectors at the wall jack, inany of the four possible locations as shown in FIG. 9. Movement of thephase reference plane enables the tester of this invention to use achannel link adapter and low cost patch cord to perform permanent linkmeasurements.

[0049] In brief, the method involves the calibration step of measuringthe overall scattering parameters S_(T) for each of the patch cords plusmated connectors pairs at each end of the patch cords, as indicated inFIG. 10. The scattering parameters S_(B) of each patch cord can beobtained from known characteristics of the cord. This, together with thetotal scattering parameter matrix S_(T) allows calculation of thescattering parameters S_(A) and S_(C) of the mated connector pairs atthe ends of the cords. With the scattering matrices of the matedconnector pairs S_(A) and S_(C) and the patch cord S_(B) known, thereference plane may be moved anywhere along the cord from within the LANtester to perform either permanent link or channel link tests.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a diagrammatic sketch of a LAN cabling connection from awork area to a telecommunications room.

[0051]FIG. 2 is a diagram of a prior art LAN tester with a test adapterand test jack.

[0052]FIGS. 3 and 4 illustrate a prior art LAN tester connection with apatch cord.

[0053]FIG. 5 illustrates a standard 90 meter link.

[0054]FIG. 6 illustrates the process for testing or “shooting” a linkwith LAN testers.

[0055]FIG. 7 illustrates a channel link configuration.

[0056]FIG. 8 illustrates a permanent link configuration.

[0057]FIG. 9 illustrates movement of the measurement reference planewith phase, as taught by the present invention.

[0058]FIG. 10 illustrates the LAN testers of the present invention.

[0059]FIG. 11 is a plot of drive signal and a resulting signal measuredby the LAN tester of the present invention.

[0060]FIG. 12 is an schematic diagram of the phase measurement circuitin a display unit of the present invention.

[0061]FIG. 13 is an illustration of setting the measurement referenceplane during factory calibration, according to the present invention.

[0062]FIG. 14 is an illustration of movement of the reference planethrough a mated connector pair.

[0063]FIG. 15 is an illustration of movement of the reference plane downthe patch cord, according to the present invention.

[0064]FIG. 16 illustrates how the reference plane at point 2 of FIG. 9relates to the reference plane at point 3 of FIG. 9.

[0065]FIG. 17 is an exploded perspective view of the LAN tester displayunit of the present invention.

[0066]FIG. 18 is an exploded perspective view of the underside of thetester unit.

[0067]FIG. 19 is a block diagram of the digital control circuit board ofa LAN tester unit of the present invention.

[0068]FIG. 20 is a block diagram of the analog circuit board of thepresent invention.

[0069]FIG. 21 is a detailed phase measurement block diagram of thepresent invention.

[0070]FIG. 22 is a circuit diagram of the patch cord with a shortcircuit termination.

[0071]FIG. 23 is a circuit diagram of the patch cord with anopen-circuit termination.

[0072]FIG. 24 is a diagram of the physical configuration of the LANtester display and remote units connected together to measure the patchcord physical length.

[0073]FIG. 25 is a plot of input impedance versus frequency.

DETAILED DESCRIPTION OF THE INVENTION

[0074] A schematic representation of the LAN testing system of thepresent invention is shown in FIG. 9. The testing systems includes ahand-held display unit 10, a hand-held remote unit 12 and first andsecond patch cords 14 and 16. Each patch cord comprises a first plug14A, 16A at one end, the actual cable 14B, 16B and a second plug 14C,16C at the other end. The display unit 10 has a channel link adapterboard 18 on which is mounted a first connector jack 20. The jack isexposed to the exterior of the display unit. Jack 20 can receive theplug 14A or 16A of a patch cord to form a first mated connector pair.When shooting a link, the other plug 14C, 16C of the patch cord mateswith a wall jack 22 attached to the link 24 running inside the walls.The remote unit 14 similarly has a channel link adapter board 26 onwhich is mounted a second connector jack 28. Both of the connectors 20and 28 are preferably right-angle connectors with appropriatepair-to-pair isolation. An RJ-45 jack or a Siemon terra jack for higherfrequencies are suitable. Jack 28 receives the plug 16A of the secondpatch cord to form a second mated connector pair. When shooting a link,plug 16C of the second patch cord 16 connects to a wall jack 30 on theend of the link 24. The display and remote units contain appropriateradio frequency and electronic circuitry for testing the link. Thedisplay unit also has user-actuated switches for starting andcontrolling the testing functions, as well as a display thatcommunicates to the user whatever data is appropriate. The display unitalso has a computer processor for performing the calculations describedbelow, and memory to store measured scattering parameters and otherdata.

[0075] Operation of the LAN testing system is as follows. First a fieldcalibration with the display and remote units and both patch cords mustbe performed. The object of this calibration is to set a measurementreference plane for the display unit and the remote unit by using anytwo patch cords with a set of channel link adapters connected to thedisplay and remote units as shown in FIG. 10. The two patch cords shouldbe made by the same vendor with identical plugs on each end, but they donot have to be the same length.

[0076] Scattering Parameters

[0077] Since the display and remote units can measure phase, thecomplete patch cord consisting of the patch cord plugs and the patchcord itself can be measured or characterized by measuring theirfrequency response with scattering, or [S] parameters. From factorycalibration, the measurement reference plane on the channel adapterprinted circuit board will be at the input to the right-angle connectorjacks 20, 28 on the channel link adapter boards 18, 26.

[0078] Measurement Steps:

[0079] 1. Connect patch cord 14 between the two units.

[0080] 2. Measure all four scattering parameters of the first patch cord14, so connected, including the mated connector pairs 20,14A and 28,14Cat each channel link adapter board 18, 26.

[0081] 3. Save the total, measured scattering data [S_(T)]₁ for thefirst patch cord 14

[0082] 4. Connect the second patch cord 16 between the two units.

[0083] 5. Measure all four scattering parameters of the second patchcord 16, so connected, including the mated connector pairs 20,16C and28,16A at each channel link printed circuit board 18, 26.

[0084] 6. Save the total, measured scattering data [S_(T)]₂ for thesecond patch cord 16.

[0085] Calculation Steps

[0086] 1. The elements for the scattering matrix, for each of the patchcords, are a set of simple equations or terms, with known formulation asfollows:

[0087] 2. To an acceptable degree of accuracy, the patch cordcharacteristic impedance Zo is known, and to a very good, first orderapproximation, may be considered to be Zo=100 Ohms.

[0088] 3. The electrical length of the patch cord will be known. Thelength may be specified by the manufacturer of the tester units, or itcan be measured by the LAN tester.

[0089] 4. To an acceptable degree of accuracy, the scattering matrix forthe mated jack and plug at each end of the patch cord can be assumed tobe identical.

[0090] 5. Then, using the justifiable assumptions, 1-4 above, and[S_(T)], the measured total scattering matrix for the first patch cord14, the scattering matrix for the mated jack and plug pair at each endof the patch cord can solved for.

[0091] 6. With the mated connector pair scattering matrix and thescattering matrix for patch cord 14, the measurement reference plane maybe moved through the mated connector pair on the printed circuit board.This reference plane location is necessary to perform a channel linktest; or the reference plane may be moved further down the patch cord towithin 1 or 2 centimeters of the wall jack, in order to perform apermanent link measurement.

[0092] 7. The same set of measurements and calculations are then madeusing the second patch cord 16.

[0093] 8. The scattering parameters for the mated connector pairs aresaved for testing throughout the day or until another patch cord set isselected, at which time the field calibration procedure is repeated.

[0094] The scattering parameter matrices can be manipulated using linearalgebraic calculations to solve for the elements of the mated LANchannel connector scattering matrix. In this set of calculations, a setof scattering parameters for the mated connector pair is assumed, andfollowing established formulation, the complete, total scattering matrix[S_(T)] is calculated by combining the scattering matrices of the matedconnector pair with that of the patch cord transmission line.

[0095] Then, with [S_(T)] as the “given” final, measured result, andwith the assumptions for the patch cord transmission line, assumptions 2and 3 above, plus assumption 4 which assumes identical scatteringmatrices for the two mated connector pairs, the program solves for theelements of the mated connector pair scattering matrix [S_(A)].

[0096] The program solves for and calculates the same values for [S_(A)]as were assumed in the original calculation for the total [S_(T)]matrix. This calculation confirms the mathematical model as correct.

[0097] Turning now to a consideration of the phase measurement aspect ofthe invention, the LAN tester of the present invention measures therelationship between two signals as it tests LAN cabling for compliancewith published LAN cabling performance standards. The signalrelationships measured by the tester are ratios in magnitude, andinclude the phase relationship between the two signals. Note that thisphase measurement under discussion is the phase between a drive signalvoltage and the corresponding coupled or reflected voltage due to thatsame drive signal. These two signals are measured at a specifiedreference plane determined by factory or field calibration procedures.

[0098] Phase difference can be shown between two sinusoidal signals atthe same frequency. In the plot shown in FIG. 11, the V_Drive trace (thesolid line) corresponds to the drive signal into the LAN cabling. TheV_Meas trace (the dotted line) is the resulting signal to be measured bythe LAN tester. Note that the amplitude of V_Meas is 40% of theamplitude of V_Drive. V_Meas also lags V_Drive by 30 degrees of phase.This lagging phase relationship between V_Drive and V_Meas can also beseen in the plot.

[0099] If the ratio of V_Meas to V_Drive is calculated, one thencalculates, for example, the crosstalk term relating the drive signal onone LAN cable pair and the coupled, crosstalk signal which appears onanother LAN conductor pair. When the ratio, V_R=V_Meas/V_Drive iscalculated, the |V_R|, the magnitude ofV_R=|V_R|=|V_Meas|/|V_Drive|=0.4/1.0=0.4. Thus |VR|=0.4.

[0100] The phase between the two signals must be calculated using one ofthe signals for the phase reference. In this case, the V_Drive signal isdefined to be the reference signal. The phase relationship of V_Meas isthen said to lag the reference signal, V_Drive, by 30 degrees of phase.Since a phase angle is involved, the ratio, V_R=V_Meas/V_Drive is acomplex number, with a corresponding magnitude, |V_R| and phase angle,φ_R=−30 degrees. The negative sign on φ_R indicates that V_Meas lagsV_Drive by 30 degrees in phase. Thus V_R=0.4/_(—)−30 degrees.

[0101] Phase may also be calculated from the time relationship of twosquare wave signals, by computing the time difference between twocorresponding edges of the square wave signals. This is illustrated inFIG. 12 where the signals travel from left to right. Note in FIG. 12 thetwo square waves, V_Meas and V_Drive, where the leading edge of V_Measlags the leading edge of the reference V_Drive square wave by the timedifference, Δt. This time difference may be used to calculate the phasebetween the two signals, by relating Δt to the period, T_(Clock), of aprecise reference clock running at frequency, F_(Clock).

T _(Clock)=1/F _(Clock)

[0102] The phase in degrees, φ_R, between these two square waves, isthen:

φ_(—) R=360×(Δt/T _(Clock)) degrees

[0103] The phase measurement circuit determines the value for Δt, andoutputs a signal related to the phase between the two square waves,V_Meas and V_Drive. The LAN tester of the present invention uses aprogrammable gate array to measure Δt.

[0104] The LAN tester can measure phase, which needs to be referenced toa measurement reference plane, as discussed below.

[0105] 1. Set the Measurement Reference Plane Initially—Duringcalibration, the phase measurement capability permits the LAN tester toset, or define a measurement reference plane at one specified pointalong the LAN link to be measured. This reference plane, defined duringthe factory calibration procedure, may be set anywhere along the link atany point, to permit measurements which are simple and convenient tomake. The calibration procedure is shown in FIG. 13.

[0106] The reference plane location is defined or set during initialfactory calibration at the display and remote ends with the procedureshown in FIG. 13. Plugs containing short-circuit, open-circuit andterminations are applied in sequence to the jack on the channel linkadapter. Swept frequency measurements are taken with each plug connectedto the jack. From the measured data, which includes phase information,the display or remote end sets its reference plane at the point lookinginto the jack on the CLA printed circuit board shown with the dottedline. With this reference plane set at this point, phase informationalso allows it to be moved from this point, up and down the patch cord.

[0107] 2. Move the Measurement Reference Plane—Following patch cordfield calibration, phase also allows the LAN tester to easily move thisoriginal calibration reference plane, during link testing. Phase allowsthe original reference plane to be moved to a new reference planelocation at any time during the LAN link testing. Specifically, withphase information, a display end, and/or remote end can each move theirphase reference plane from within the channel link adapter PCB, throughthe mated pair of connectors at the CLA output, anywhere down the lengthof the patch cord and up to the mated pair of connectors at the walljack, in any of the four possible locations as shown in FIG. 9.

[0108] 2a. Reference plane movement through the mated connector pairfrom 1 to 2, shown in FIG. 14, on the channel link adapter module isperformed using the [S₂₁]_(Connector Pair) data measured and calculatedfor the mated connector pair during patch cord field calibration. Thisstep sets the reference plane for channel link testing.

[0109] 2b. Reference plane movement down the patch cord from 2 to 3,shown in FIG. 15, moving down the patch cord is performed using the[S]—parameter data measured and calculated for the patch cord duringfield calibration.

[0110] Note that the desired length down the patch cord, L Line, from 2to 3 is expressed in the physical length units of inches. L_(Line), ininches, needs to be converted into equivalent electrical phase length,φ_(Line) in degrees.

[0111] During the patch cord field calibration, the NVP (nominalvelocity of propagation) for the patch cord is determined throughmeasurement. From this value, the corresponding electrical phase length,φ_(Line), moving from 2 to 3 is calculated using:

β=(360×f)/(NVP×c) degrees/inch

[0112] where:

[0113] c velocity of light in freespace=1.1811×10¹⁰ inches/second

[0114] f=signal frequency in Hertz

[0115] Then φ_(Line) in degrees is calculated using:

φ_(Line) =L _(Line)×(360×f)/(NVP×c) degrees

[0116] Relating this formulation to just the patch cord referenceplanes, moving from 2 to 3 can be seen in FIG. 16.

[0117] The measured LAN cable data is moved thru the mated connectorpair from 1 to 2 using the [S₂₁]_(Connector Pair) data as shown in FIG.14. With reference to FIG. 15, which shows how a patch cord length ininches is related to the equivalent patch cord electrical length indegrees, FIG. 16 relates these terms per the formulation below:$\lbrack S\rbrack_{Patchcord} = \begin{bmatrix}S_{11P} & S_{12P} \\S_{21P} & S_{22P}\end{bmatrix}$

[0118] For a reasonably well-matched patchcord, this expression becomes:$\lbrack S\rbrack_{Patchcord} = \begin{bmatrix}0 & ^{{- j}\quad \varphi \quad {Line}} \\^{{- j}\quad \varphi \quad {Line}} & 0\end{bmatrix}$

[0119] If the patch cord has characteristic impedance, Z_(0p), not equalto

[0120] Z₀=100 Ohms, then the patch cord S_(11p) and S_(22p) are non-zeroand are then replaced by non-zero values calculated by using standardtransmission line theory.

[0121] Finally, the measured LAN cable data with reference to plane 2,is related to plane 3, through the use of, [S]_(Patchcord), the patchcord scattering matrix. $\lbrack S\rbrack_{Patchcord} = \begin{bmatrix}0 & ^{{- j}\quad \varphi \quad {Line}} \\^{{- j}\quad \varphi \quad {Line}} & 0\end{bmatrix}$

[0122] From this matrix it can be seen that for a well-matched patchcord, the usual case, there is no effect upon the S₁₁ and S₂₂ matedconnector pair terms. The only effect to the mated connector pairscattering matrix is the added phase term, e^(−j{acute over (ω)}Line).

[0123] Thus, with the patch cord NVP known by measurement orspecification, the measurement reference plane may be moved through themated connector pair on the channel link adapter board and down thepatch cord a specified number of inches from plane 2, at the output ofthe mated connector pair.

[0124] Turning now to a more detailed description of the tester units,the exploded views of FIGS. 17 and 18 show the overall physicalconfiguration of the tester and show how its printed circuit boards arehoused. The tester shown is a display unit 10. It will be understoodthat the remote unit is similar. The tester has a housing including afront enclosure 32 and a rear enclosure 34. The rear enclosure defines areceptacle or well 36 for receiving and mounting a channel link adapterprinted circuit board 37. Inside the housing there is a digital controlmodule 38 that drives and controls an analog stimulus/measurement module40. Both modules are built into printed circuit boards and will bereferred to herein as the digital board and the analog board. The analogboard 40 includes a connector 42 on the underside thereof. Thisconnector is releasably engageable with a mating connector on thechannel link adapter through an opening 43 in the rear enclosure at thebottom of the well 36. A time domain reflectometer (TDR) 44 measurementcapability is provided by a third separate module. Other componentsshown in FIG. 17 include a PCMCIA card holder 46, a universal serial bus(USB) port 48 and a serial port 50. These are mounted on the digitalboard 38. A color display unit 52 and a keyboard 54 are mounted on or inthe front enclosure 32. Further details of the physical arrangement ofthe housing may be as shown and described in U.S. patent applicationSer. No. 09/863,810, filed May 22, 2001 entitled “Apparatus withInterchangeable Modules for Measuring Characteristics of Cables andNetworks”, the disclosure of which is incorporated herein by reference.

[0125] The overall function of the digital control module 38 is shown inthe digital control circuit block diagram of FIG. 19. The digital boardis controlled by a high-speed central processing unit (CPU) 56 driven bythe firmware installed within the tester. Several memory blocks (notshown) may be provided, as well as a RAM memory 58, a small boot flashmemory 60, and a larger boot flash memory 62. The tester communicateswith an external personal computer (PC) 1 either by using the USB 48, orwith a serial interface connection 50 to the CPU 56. Flash memory ornetworking cards 64 can be installed in the tester, which connect to theCPU through the PCMCIA block 46. These cards can be used to storeadditional test results, or to upload new firmware to the CPU. Otherconnections to or from the CPU include the keyboard 54, the colordisplay 52, a speaker phone 66, a real time clock 68 and temperaturesensors 70 to compensate the analog board performance as temperaturerises.

[0126] Of utmost importance is communication with the analog board 40through the I/O bus 72. This bus is shown as a separate block because itinterfaces control commands to the analog board 40 from the digitalboard 38, and it returns measured data from the analog board for storagein the display unit memory and for display on the color display 52.

[0127] The LAN tester analog circuit block diagram of FIG. 20 shows themajor functional blocks on the analog board 40. Other blocks have beenomitted for clarity. The analog board generates a set of continuouslyvarying low frequency (LF) and radio frequency (RF) signals which areapplied to one selected conductor pair of the LAN cabling through theuse of signal switching relay banks 74 on the analog board. The samerelay banks carry the return signal to be measured from another selectedLAN cable conductor pair back into the analog board. Circuit blocks onthe analog board then condition the return test signal and measure itscharacteristics relative to the applied drive signal. The low frequencyLF measurements include cable capacitance, length, conductor wire DCresistance, wire mapping and delay. The circuit blocks associated withthese lower frequency signals are identified by the associated notation.

[0128] Note the notation of “MUX” several places in the block diagram. AMUX is shorthand notation for a multiplexer, which is a switching devicethat routes an input of several different signals to a selected signalpath. The LAN tester analog board uses several MUX's since it is afour-channel test instrument, capable of testing two of the possiblefour conductor pairs of the LAN cable under test. The MUX's are requiredfor signal routing and channel to channel signal isolation.

[0129] Circuit blocks 76, 78 are shown for RS-485 blocks, one forcommunication, and another for LAN tester interface with the gate arrayand measurement IC 80, and for DC power control and power management onthe analog board.

[0130] The analog board 40 also measures the RF parameters of cablecrosstalk, return loss and attenuation. Specifically the analog boardmeasures the ratio of the amplitude of the returned signal divided bythe amplitude of the RF drive signal. Circuitry has been added to theanalog board in the unit to measure the phase of the returned testsignal relative to the RF drive signal sent out on the selectedconductor pair.

[0131] The RF drivers 82 send a signal from the RF synthesizer 84 out onone pair of the LAN cable conductors via the RF signal switching relays74. The drive signal is also sent to the return-loss bridges 86.

[0132] The resulting test signal comes into the tester via the same setof RF relays 74 and is routed through the return-loss bridges 86 to theRF mixer block 88. There it is mixed with the local oscillator (LO)signal and converted into the test IF (intermediate frequency) signal.

[0133] As mentioned above, the RF drive signal is also sent into thereturn-loss bridges 86. As shown in FIG. 20, the drive signal sent tothe return-loss bridges enters the mixer 88 and is converted to a phasereference IF signal. All IF signals associated with the LAN measurementare compared with this phase reference IF signal to determine the phaseof that measurement signal.

[0134] Once both the Ref IF and Test IF signals have been created theyare delivered to the phase detector 90 and the reference and testmagnitude detector blocks 92 and 94. The phase detector block 90 sendsthe phase information into the gate array and measurement IC 80. Theoutputs from the reference and test magnitude detectors 92, 94 are sentto the analog-to-digital (A to D) Mux 96 and then to the A to Dconverter 98. From there the magnitude ratio signal is sent to the gatearray and measurement IC (integrated circuit) 80.

[0135] The gate array and measurement IC 80 finishes the computation ofthe phase between the test and reference IF signals, and the ratio oftheir amplitudes, to formulate a complex number representation of themeasurement. Output from the IC 80 is placed on the analog I/O bus 72,which communicates with the digital board 38. Thus, the phasemeasurement function for the testing unit is controlled from the digitalboard, but is measured and computed on the analog board. The measurementresults are then carried to the digital board from the analog board.

[0136] The LAN tester phase measurement block diagram of FIG. 21 showsthis function on a phase measurement system level. This block diagramalso shows computation of the magnitude ratio on the analog board. TheI/O bus 72 carries the control signals from the digital board to theanalog board, and it also carries both the phase and magnitude of thetest signals to the digital board. Once the test signal has beendelivered to the display board it can be stored in the memory or shownin color graphic form, plotted on the display screen 52.

[0137] Regarding measurement speed, the tester architecture has beendesigned in such a manner that a LAN cable conductor pair may be drivenwith RF signals from either the display or remote test units 10 or 12.All other non-driven lines may then be simultaneously connected to themeasurement circuitry via the MUX circuitry on the analog boards withineach unit. This design feature provides for significantly reduced testtimes, while still providing measurement of test signal magnitude andphase.

[0138] Attention will now be turned to a discussion of how the foregoingmethods and apparatus can be used to measure the physical length of thepatchcords. The following procedure describes an application which maybe used to accurately determine the physical length of patch cords usedby LAN field testers. As explained above, when LAN field testers areused to certify commercial or residential LAN cabling installations,patchcords connect the field tester to the cabling under test. Theeffects of the path cord physical length, and associated electricallength, must be included within the measurement of these links. Beforetesting any cabling link, the field tester must be calibrated. Part ofthe calibration is done in the factory as a required step in thefabrication of the field tester. The second calibration step, done inthe field before testing the link cabling, characterizes the pair of LANtester patch cords. During field calibration, each patch cord ischaracterized for line loss or attenuation and return loss.

[0139] The patch cord physical length may also be easily determinedduring the field calibration procedure by sweeping the patch cord,terminated at the other end by either a short-circuit load or anopen-circuit load, over a range of frequencies. As the frequency isswept, the field tester measures the input impedance looking into thepatch cord at the reference plane. From the measurements of |Z_(in)|,the magnitude of the input impedance looking into the patchcord, thefield tester can determine the physical length of the patch cord.

[0140] The equivalent circuit for patch cord with a short circuittermination is shown in FIG. 22. The equivalent circuit for the patchcord with an open-circuit termination is shown in FIG. 23. These circuitmodels will be used to calculate the results in the followingdevelopment of the theory used to determine the patch cord physicallength.

[0141]FIG. 24 shows a pair of LAN field tester display and remote units10 and 12 connected together during the part of field calibrationperformed to measure the patch cord physical length. For discussionpurposed, it is assumed that the display end patch cord 14 is shownconnected between the LAN field tester display unit 10 and the remoteunit 12. When the patch cord physical length is to be measured by thedisplay unit, the remote unit places either a short-circuit oropen-circuit termination across each of the pairs of the LAN cabling byusing electrical switches.

[0142] As the frequency is swept, the display unit 10 measures the inputimpedance looking into the patch cord 14 at the reference plane. Theinput impedance plot, shown in FIG. 25, illustrates typical inputimpedance versus frequency for a patch cord of two meters in physicallength. The two meter physical length is a typical length for patchcords used by technicians as they certify a LAN cabling link duringfield testing and certification. It will be understood that the twometer length is noted here for illustrative purposes only. The actuallength is, of course, not known ahead of time.

[0143] Whether one measures the patch cord input return loss for anopen-circuit load, or a short-circuit load, the maxima in the resultinginput impedance magnitude plot are sharply defined in frequency, risingrapidly up to the maximum value and dropping sharply to near zerobetween maxima. Note that the magnitude of |Z_(in)|, the magnitude ofthe input impedance looking into the patchcord, reaches values of 10,000ohms or greater for frequencies up to 150 MHz in the plot shown in FIG.25.

[0144] The key point of this procedure is that the maxima are exactlyone half wavelength apart, from one sharp maximum ‘spike’ to another.This principal holds for any open circuit or short-circuitedtransmission lines. Since patch cords are transmission lines, thisprincipal also applies to them. In addition, this principal holds forall patch cords and transmission lines with small or moderate levels ofline losses.

[0145] The associated pattern plotted in FIG. 25 occurs for anytransmission line or patch cord not terminated in the patch cord'scharacteristic impedance, Z_(cord). However, the maxima in the impedancemagnitude plot are more sharply defined when the patch cord isterminated in either an open-circuit or short-circuit load.

[0146] Calculation of the patch cord physical length is as follows. Thepatch cord physical length L_(cord) is directly related to the frequencydifference Δf, between the maxima in the input impedance magnitude plot.After measurement of this frequency difference, the display unitcalculates L_(cord) from the following:

L _(cord)=λ/2=(NVP*c)/(Δf*2) meters

[0147] where: NVP is the propagation constant for the patch cord. Thevalue for NVP is determined by the display unit in an earlier fieldcalibration step as described above. c is the velocity of light in avacuum, 3×10⁸ meters/second.

[0148] Δf is the frequency difference between two maxima in the inputimpedance magnitude plot.

[0149] In this instance, from the plot Δf=54 MHz=54×10⁶.

[0150] From patch cord measurements, NVP=0.72.

L _(cord)=(0.72*3*10⁸)/(2*54×10⁶) meters

L _(cord)=(216)/(108) meters=2 meters

[0151] This procedure would then be repeated during calibration for thepatch cord associated with the remote unit. In this case the remote unitwould drive this patch cord and the display unit would terminate theconductor pairs in open-circuit or short-circuit loads. The remote unitwould then determine the physical length of this patch cord by the samecalculation.

[0152] This procedure works for either open-circuited or short-circuitedpath cords, since all that is required is Δf, the frequency differencebetween the maxima, and the NVP for the patch cord, a value determinedearlier in the field calibration procedure. Lossless transmission linesor lossless patch cords are not required in order for this method todetermine the patch cord physical length. This procedure also works forreal-world patch cords that may have moderate or low line losses.

[0153] While a preferred form of the invention has been shown anddescribed, it will be realized that alterations and modifications may bemade thereto without departing from the scope of the following claims.

I claim:
 1. A LAN cabling testing system, comprising: first and secondpatch cords each terminating at first and second plugs; a hand-helddisplay unit and a hand-held remote unit, each one of said unitsincluding means for sending and receiving a wave form of selectedfrequency to and from the other of said units through said patch cordsand a LAN link to be tested; the hand-held display unit including a jackfor receiving a plug of one of the patch cords, said jack and plugdefining a first mated connector pair; the hand-held remote unitincluding a jack for receiving a plug of the other of the patch cords,said jack and plug defining a second mated connector pair; phasemeasuring means for measuring phase in one of the display or remoteunits and for setting a reference plane at one end of the patch cord;means for measuring the input impedance looking into the patch cord atsaid reference plane; means for measuring the separation Δf betweenmaxima of a plot of the input impedance versus frequency; and means forcalculating the patch cord length according to the relationL_(cord)=(NVP*c)/(Δf*2) meters.
 2. In a LAN cabling testing system ofthe type having a display unit and a remote unit and first and secondpatch cords of unknown lengths L1 and L2, the patch cords eachterminating at first and second plugs, and the display and remote unitseach having a jack for receiving a patch cord plug, a plug and jack whenconnected comprising a mated connector pair, the display and remoteunits each having means for sending and receiving a wave form ofselected frequency to and from the other unit through said patch cordsand a LAN link to be tested, a method of measuring the length of a patchcord comprising the steps of: connecting the patch cord to the displayunit and to the remote unit; determining the nominal velocity ofpropagation, NVP, of the patch cord; at one of the display and remoteunits, sequentially placing one of a short-circuit or an open-circuittermination across each of the wire pairs of the patch cord; sweepingwave forms across the patch cord in a range of frequencies; measuringthe input impedance looking into the patch cord at a reference plane atone end of the patch cord; measuring the separation Δf between maxima ofa plot of the input impedance versus frequency; and calculating thepatch cord length according to the relation L_(cord)=(NVP*c)/(Δf*2)meters.
 3. A method for determining the physical length of a patch cordused with the display unit and remote unit of a LAN field tester,comprising the steps of: connecting the patch cord to the display unitand to the remote unit; determining the nominal velocity of propagation,NVP, of the patch cord; at one of the display and remote units,sequentially placing one of a short-circuit or an open-circuittermination across each of the pairs of the patch cord; sweeping a waveform across the patch cord in a range of frequencies; measuring theinput impedance looking into the patch cord at a reference plane;measuring the separation Δf between maxima of a plot of the inputimpedance versus frequency; and calculating the patch cord lengthaccording to the relation L_(cord)=(NVP*c)/(Δf*2) meters.